A genuine "one way" mirror is impossible to build because it violates the laws of thermodynamics. If you made up a box of "one-way" mirror material which was oriented so more light flows into the box than flows out, then more and more light would flow into the box and build up there. It takes energy to do such a thing. If genuine "one-way" mirrors existed, they would serve as energy sources, and you could use one to build a Perpetual Motion machine.

I could be wrong as I've only had one year of college level physics, but I disagree with this last paragraph. Yes, a one way mirror cannot be created -- but not for this reason. This would not be a perpetual motion machine. First of all, a perfect mirror cannot be created, because if it were, it would have to be perfectly (infinitely) smooth, but on the atomic level such a surface cannot exist. When light hits a mirror, some energy is absorbed by exciting (heating and moving) the atoms of the mirror. Therefore the light would not be perfectly contained in the box.

"It takes energy to do such a thing." Ummm, the light is the energy.

"Light would flow into the box and build up there.” How is this different from put a piece of black paper in the sunlight and watching it heat up? Did I just create energy?

P.S. IANAP